1. Field of the Invention
The present invention is directed to an antenna for a magnetic resonance apparatus having an antenna conductor that can be adapted to examination regions of different sizes.
2. Description of the Prior Art
Local antennas are utilized in a diagnostic magnetic resonance apparatus for the examination of sub-regions (i.e., a body portion) because they have a better signal-to-noise ratio compared to a whole body antenna. It is thereby meaningful to adapt the local antenna to the body part or to the examination region to be examined as exactly as possible in order to achieve a high filling factor, and thus to optimize the signal-to-noise ratio of the antenna.
U.S. Pat. No. 4,897,604 discloses an antenna of the type generally described. This known antenna is fashioned as a frame antenna that embraces an examination volume. The conductor of this frame antenna has a gap into which conductor bridges of different lengths can be inserted. The conductor bridges can comprise a capacitor whose capacitance compensates the inductance of the conductor bridges at an operating frequency of the magnetic resonance apparatus. The resonant behavior of the antenna is therefore minimally influenced by the inserted conductor bridges. The tuning and matching circuit required for the tuning of the antenna can consequently cover the same tuning range as antennas that are not adaptable or variable in size. Manipulation of this known antenna in the adaptation to examination regions of different sizes, however, is complicated. A number of conductor bridges of different lengths must be kept on hand, a conductor bridge having a suitable length having being selected therefrom.
German OS 42 21 759 discloses an antenna for circularly polarized high-frequency magnetic fields, composed of a combination of two sub-antennas whose antenna characteristic or magnetic axes reside perpendicularly relative to one another. The first sub-antenna is thereby fashioned as a frame coil and the second sub-antenna is fashioned as two saddle coils arranged lying opposite one another. The antenna is partly flexible and can therefore be brought into intimate contact with a patient. The antenna can be adapted to examination regions of different sizes by interchanging coil parts.
The antenna conductor of a local antenna disclosed in European Application 0 233 211 can also be adapted to examination regions of different sizes. The antenna conductor therein is glued on a thin, flexible carrier of electrically insulating material. The material is adequately flexible in order to wrap the antenna tightly around an examination region. The carrier material has fastening means in order to detachably secure the antenna to the examination region. A disadvantage of this known antenna is that the inductance, and thus the operating frequency of the antenna, change given a variation of the diameter of the antenna. In order to be able to operate the antenna at an unchanging operating frequency, a large swing (adjustment range) for the capacitors must be provided in the tuning or matching circuit. This requires a high circuit outlay.
European Application 0 396 804 discloses a local antenna having a conductor arrangement that can be interconnected to different active coil segments in the form of reception loops having different sizes and/or different positions. Each end of each possible reception Iccp is thereby connected to a signal reference line via a first controllable capacitor and to a signal line connected to an evaluation unit via a second controllable capacitor. This arrangement makes it possible to activate individual coil segments, to tune them to resonance, and match them to an impedance of the feeder or of the evaluation unit.
German OS 33 23 657 discloses an antenna wherein the geometry of the antenna conductor loops is variable.